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post #1 of 30 (permalink) Old 09-12-2011, 10:11 AM Thread Starter
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Engine Temperature Issue - Please Help!

Out of no where (at 154,000 miles), I noticed that in 100 degree temperatures, my engine temperature gauge that normally hits the halfway mark and stays there rather quickly (like within a couple minutes of running), started fluctuating and showing cool and sometimes nearly on cold. I was getting erratic (never hot) temperature gauge readings over the weekend. Once I would drive on the highway or have it running for a good while, it would rise to the midway mark like normal but if I were to go into say the convenience store (keeping in mind it's 100 degrees out) and then come back out after 5 minutes and start it back up, the gauge is back down there near the damn cold line. Obviously, after having the car for 154,000 miles, I know this isn't the case, the engine is blazing hot like always.

Ok so things to most likely rule out:

-Air bubble in coolant. Yes, I had changed two coolant hoses a couple weekends ago as preventative maintenence. Of course I purged a lot of old coolant in the process but filled up the cooling system and burped the car like I have done before. Additionally, the car has been driven about 1,000 miles since then and checked for air in the radiator and topped off like I have done numerous times previously.

-Bad Engine Coolant temperature sensor. This part is brand-new as a month ago. Keeping in mind that my old ECT sensor was completely busted (as in the top connector part spun freely independently of the metal nut part that screws into the head). Even when my old ECT sensor was bad, I was still getting the engine temperature to read correctly. The engine temp gauge must get a signal from some other temperature sensor.

-Thermostat. The engine had a new thermostat installed when I had the new engine installed at 109,000 miles. It's a factory Mazda thermostat. Additionally, if a thermostat was stuck open, I don't see how the coolant could be at the midway point (according to the gauge) and then after being shut off for 5 minutes, show COLD.

Now, one piece of additional info. is that my check engine light did come on once during this little mystery and the code was for "insufficient coolant temperature for stable operation" which makes sense. So the question becomes, is there another sensor somewhere that sends the engine temperature to the gauge? It must not be the ECT sensor because mine was completely broken in the past and my temperature gauge still functioned correctly.

I would definitely appreciate some help guys. I'll also confirm, with my AP, that my ECT is around 200 like it normally is. I would expect the ECT will be 200 even though the gauge will read cool or even COLD.

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Last edited by mczimer; 09-12-2011 at 12:28 PM.
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post #2 of 30 (permalink) Old 09-12-2011, 01:21 PM
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Fluctuating temp gauge is usually a sign of bubbles in the coolant.

Since it's the easiest thing, try burping the system again. Sometimes it's easier to open the radiator cap (with the car cold of course), jack up the front end as high as possible (so the fill location is the highest point), start the car and let it idle until the rad fans kick on 2-3x, filling when necessary.
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post #3 of 30 (permalink) Old 09-12-2011, 03:10 PM Thread Starter
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Quote:
Originally Posted by SJP0tato View Post
Fluctuating temp gauge is usually a sign of bubbles in the coolant.

Since it's the easiest thing, try burping the system again. Sometimes it's easier to open the radiator cap (with the car cold of course), jack up the front end as high as possible (so the fill location is the highest point), start the car and let it idle until the rad fans kick on 2-3x, filling when necessary.
Thanks and yes that does make sense. The thing that makes me wonder though is that I did the coolant hoses a couple weeks ago and have driven the car at all speeds for about 1,000 miles since then and now it has just started doing this.

Do we know where else the engine temp is read? There has to be another place other than the ECT sensor that's screwed into the head near the side of the battery area.

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post #4 of 30 (permalink) Old 09-12-2011, 03:46 PM
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I'm fairly certain there's the temp sensor from which the ECU receives input, and a type of sending unit from which the gauge in the cluster operates...
Don't you have a scangauge or Dash Hawk or AP? Have you checked the temps there?


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post #5 of 30 (permalink) Old 09-12-2011, 04:47 PM Thread Starter
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Quote:
Originally Posted by IHeartGroceries View Post
I'm fairly certain there's the temp sensor from which the ECU receives input, and a type of sending unit from which the gauge in the cluster operates...
Don't you have a scangauge or Dash Hawk or AP? Have you checked the temps there?
I've got the AP, I can check the actual temperatures with that. I just hadn't had a chance to do it yet. This happened while I was out of town over the weekend and just got back yesterday evening. At work at the moment.

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post #6 of 30 (permalink) Old 09-12-2011, 05:06 PM
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Im thinking that if its not a bubble issue then maybe you have a bad temp sensor. I would check it and see.

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post #7 of 30 (permalink) Old 09-12-2011, 08:37 PM
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While I've read on 6 club multiple times in the past about a sending unit, I could not verify this in the WSM. Was your error code P0126/P0128?
I would speculate that if IAT remains below a threshold (normal), but ECT signal is erratic or indicates high temp, the ECU does not "see" a high ECT, but instead determines a mechanical malfunction, so your gauge does not display a high reading.

WSM indicates the following:

"DTC P0126:
If the ECT signal never exceeds 160 degrees F. after engine is started for a specified period, the PCM determines that the coolant thermostat is stuck open.
DTC P0128:
PCM monitors MAF, IAT, VSS, and ECT signals and calculate radiator's heat the radiation ratio when the following monitoring conditions are met. If the calculated value exceeds the threshold, the PCM determines that the cooling thermostat is stuck open."


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post #8 of 30 (permalink) Old 09-13-2011, 10:59 AM Thread Starter
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Ok so here is what I noticed today. I plugged in the AP and brought up the coolant temperature. The temperature gauge came up to the middle range and was holding and then coolant was at about 180 degrees. Keep in mind this was in the morning from cold start and I was just loafing down the street to the gym so it hadn't climbed to the usual high 190s or over 200 like it does after some agressive highway driving. As I cruised down the street I noticed very erratic readings on the AP. It was jumping all around from 136 to 175 then to 160 then to 150 then back up to 180. I noticed my engine temp needle was dipping down and then came back up and stabilized again. Clearly something is giving/receiving an erratic signal.

Now just a recap, when my ECT (engine coolant temperature) sensor was completely broken (I discovered the top connector part broken free from the actual sensor in the head), I was still having full operation of my temperature guage on my dash. This tells me it has to be receiving it's signal from something else.

Additionally, the ECT sensor is brand-new as of about a month ago. It's straight from Mazda, OEM and all. I thought I had heard a while back that there was another sensor from which a temperature reading is taken but who knows if it was accurate.

On the flipside, if the ECT sensor that is in the head isn't used to send the engine coolant temperature to the gauge, then what is it used for?! LOL Frustrating little mystery.

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post #9 of 30 (permalink) Old 09-13-2011, 12:56 PM
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Many cars have 2 sensors: One for the crappy analog in-dash gauge, and one that's much more accurate for the ECU to use, which is what the AP would read.

If it's jumping around like that it still sounds like air bubbles. I've seen thermostats be flaky & stick open/closed only sometimes, but your symptoms don't really match that behavior.
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post #10 of 30 (permalink) Old 09-13-2011, 01:18 PM
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Agreed alot have a sending unit for gauge and separate ECT sensor for ECU input. But I could not verify this in this case in the WSM.
Here's my theory:
With the connector broken and the ECU receiving no input from the sensor, the ECU then utilizes input from IAT. With this input and perhaps input from others, a calculation is made and the gauge displays accordingly.
With a bad or faulty ECT sensor, or a problem with connection or circuit, the erratic readings are causing your gauge to fluctuate.

The ECT sensor is just resistance based like most other temp sensors on the car. You can test it yourself easily...


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